Add specific id to responsive css selector
Add specific id to responsive css selector

A specific id was missing from one of the responsive
css selectors. This is probably not on purpose.
Without this id many settings in the admin menu only
uses half the width when viewed on screens with widths
below 480 px.

Signed-off-by: Oscar Rydhé <2b3e17a7d6f527397936131db0874b14455019f5@gmail.com>

StealthWombat/SMF2.1-more-forkery,StealthWombat/SMF2.1-more-forkery